Because of its high efficiency and fidelity, digital data transmission is becoming widely used in such areas as wireless communication, broadcasting, etc. During the transmission, data is encoded in one or more carrier wave signals using a modulation scheme, e.g., amplitude modulation, frequency modulation, phase modulation, pulse width modulation. At the receiver, the modulated carrier wave signals are demodulated and the data is decoded there from.
For example, High Definition Television (HDTV) broadcasting often uses Quadrature Phase Shift Keying (QPSK) and Quadrature Amplitude Modulation (QAM) schemes. In order to demodulate the carrier wave signals and decode the data, a receiver must be synchronized to the transmitter in data transmission rate, generally referred to as baud rate. This is generally achieved by a timing recovery loop. In one baud rate synchronization approach, a down converter generates a base band signal from the received QAM signal. The base band QAM signal is fed to a variable decimator controlled by a numerically controlled oscillator in the timing recovery loop. The variable decimator generates a sampling signal with a sampling rate controlled by the numerically controlled oscillator. A square root filter performs pulse shaping on the sampling signal, which is then transmitted to a timing error detector. The timing error detector generates a timing error signal indicating whether the sampling rate is synchronized with the baud rate of the received signal. The timing error signal is transmitted to the numerically controlled oscillator via a loop filter to adjust the sampling rate of the variable decimator.
The square root filter in the timing recovering loop generally has a large delay, which results in small baud rate acquisition range. In order for such a timing recovery loop acquire the target rate, the initial baud rate estimate must be relatively close to the target rate. In other words, the receiver of the QAM signal must know at least the approximate baud rate of the transmitted QAM signal in advance. Otherwise, the receiver must perform a rate sweeping process with sweeping steps equal to or smaller than the rate acquisition range. A typical timing recovery loop has an acquisition range less than 400 parts per million. For a QAM signal having a baud rate range between one and seven megabytes per second, it may take up to several minutes to acquire the target rate. This may be frustrating to a user who has to wait several minutes to view a TV program after turning on the TV set.
Accordingly, it would be advantageous to have a device and a process that can quickly acquire the target rate in digital signal transmission. It is desirable for the device to be simple and cost efficient. It is also desirable for the rate acquisition process to be able to acquire the data transmission rate over a wide range.